When a typical motion compensation algorithm is used to compress video images found in dissolve regions, fade regions, fast motion regions or complex motion regions, a high entropy prediction error image is often produced. Encoding such an error image requires more bits than usual, which may not be affordable for all images in a region without increasing the average bit-rate. Furthermore, an image that is encoded with an insufficient number of bits may appear blocky or ugly when it is decoded. Similar problems in the appearance of a decoded image may occur whenever the quantization step size used to encode the image becomes too large.
During the decoding of compressed digital video image data, an image buffer is typically interposed between the decoder and the image display. Since the actual time necessary for decoding a particular image often varies within a sequence, the image buffer is needed to ensure that a constant supply of decoded images remain available for display. Despite the use of such an image buffer, it sometimes happens that the constant supply of images to the display is interrupted when a group of images requiring above-average decoding time are presented for decoding close in time.
It is therefore an object of the present invention to provide an improved method and apparatus for encoding images in dissolve regions, fade regions, fast motion regions or complex motion regions which reduces the number of bits needed to encode such images or improves image quality for a fixed number of bits.
It is a further object of the present invention to provide an improved method and apparatus for encoding images where the quantization step size being used becomes too large.
It is a still further object of the present invention to provide an improved method and apparatus for ensuring a constant supply of decoded images to an image display.
Further objects and advantages of the invention will become apparent from the description of the invention which follows.